Due to its superior strength, epoxy is ideal for filling structural gaps. For the epoxy to stay in place and keep the desired form, it needs to be the consistency of peanut butter.
You can thicken the epoxy yourself by adding siica thickener to mixed epoxy (TotalBoat High Performance Epoxy or TotalBoat Traditional 5:1 Epoxy systems are recommended). To help fill the gap faster and easier, you can also add some microballoons to this mixture. Microballoons also make sanding easier.
Alternatively, you can use a pre-thickened product in a cartridge, such as TotalBoat Thixo (regular formula).